[rrd-users] Re: Can CDEF be made to compute averages from 2 different data sources ?

Alex van den Bogaerdt alex at ergens.op.het.net
Fri Feb 27 10:39:11 MET 2004

On Fri, Feb 27, 2004 at 12:51:24AM -0800, Eric Ho wrote:

> I've 2 different data sources from 2 separate RRD files.
> Both heartbeats are the same, and the 'step' for both RRD files are the same.
> Is it possible to write a CDEF to compute the average of some of the
> samples from these 2 data sources and then graph this average value ?
> By 'some samples', I mean samples from a certain period of time
> in the past.
> By looking at the manpages of 'rrdtool graph', I can't see how 
> this can be done via CDEF...  or am I misusing CDEF or missing
> the point of CDEF ?

You can't access RRD files using "CDEF".  Accessing original data is
done using "DEF".  When you have access to the data, so when it is in
a "vname" (DEF:vname=....) you can process it using "CDEF".

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